1. Field of the Invention
The invention relates to a CMOS semiconductor structure having a substrate, a gate oxide, a polysilicon layer lying thereover to form magnetoresistors, and further layers lying thereover, in particular an oxide layer. The invention also relates to a process for producing such a CMOS semiconductor structure, in which, on a substrate, a gate oxide is produced, a polysilicon layer, a nitride layer and an oxide layer are deposited and anisotropic etching, which is masked by using a photolithography technique, is carried out as far as the gate oxide.
During the production of integrated CMOS circuits, a considerable proportion of the area is lost to electrical contacts between source and drain regions of the transistors and a first metal plane. The surface-area requirement is due, on one hand, to a minimum size of a hole and, on the other hand, to minimum separations between structure edges of the field oxide and gate electrode and other structures. The minimum separations to be retained between those structures have to accommodate the usual misalignment tolerances associated with the process. For their part, the edges of the field oxide must in turn maintain a minimum separation from the gate electrode, so that considerable separations are found overall. Overall, that results in large chip areas and large lead resistances as well as large stray lead capacitances.
Various approaches are already known for solving that problem. Thus, in particular for periodic structures of memory arrays, the contacts are to be produced by self-aligned techniques. That technique is known by the term of art "fully overlapping bit line contact". In a different approach, an overlap of the contact hole over the edge of the field oxide is permitted and the contact regions are subsequently implanted. However, that approach requires at least one additional photolithography technique, and the problem of dopant activation must be solved.
Anisotropic etching methods have also been developed, which can etch oxide selectively with respect to nitride. Using those etching methods, it is possible to carry out contact-hole etching with self-alignment with respect to previously structured nitride regions. Nevertheless, it is difficult to achieve such high selectivity, which has the consequence of limiting the maximum depth of contacts produced in that way.